Green’s Dictionary of Slang

garter n.

1. the vagina [euph.].

[UK]G. Stevens ‘Transit of Venus’ in Songs Comic and Satyrical 151: Does th’ Empress of Joy [...] Wear Garters above, or ’low knee?

2. (US prison) an indeterminate prison sentence [SE garter, which stretches].

[UK]‘Spindrift’ in Partridge DU (1949) 279/2: A sentence of imprisonment is a rap [...] Garter is an indeterminate sentence – say from two to ten years .
[US]Goldin et al. DAUL 77/1: Garter. (Very rare) A prison term of uncertain length, with wide difference between minimum and maximum terms.
[UK]J. Morton Lowspeak.
[US]Bentley & Corbett Prison Sl. 20: An indeterminate sentence is a sentence range imposed to a convicted felon requiring the inmate to serve no less than a minimum number and no more than a maximum number of years […] (Archaic: garter).

SE in slang uses

In phrases

seek the garter (v.) (also steal the garter)

to caress sexually; to have sexual intercourse.

[UK] ‘The Winchester Wedding’ in Farmer Merry Songs and Ballads (1897) IV 50: Cause Arthur had stolen her Garter, / And swore he would tye it himself.
[UK] ‘The Winchester Wedding’ in Playford Pills to Purge Melancholy I 24: Cause Arthur had stolen her Garter, / And swore he would tye it himself.
[UK] ‘Love’s Holiday Is Come’ in Farmer Merry Songs and Ballads (1897) II 134: And in seeking the Garter, much pleasure was found.
[UK] ‘Hark the Cock Crow’d’ in Farmer Merry Songs and Ballads (1897) IV 242: Roger the trusty, tell Rachel the lusty, / The Barn’s a brave place to steal Garters.
tie a garter (v.)

to caress a woman sexually.

[UK] in Ebsworth Choyce Drollery (1876) 45: The fair maid spread her legs abroad, / The young man fell between, / Such tying of a Garter I think was never seen.
[UK] in D’Urfey Pills to Purge Melancholy : The Maid she held her Legs so wide, / The Young man slipt between, / Such tying of a Garter, / You have but seldom seen.
[US] in Randolph & Legman Ozark Folksongs and Folklore (1992) I 164: I tickled her bubbies, I patted her ass. / She felt my prick and she played with my balls, / She kicked and she farted and let out a squall. / She spread out her legs and I jumped in between, / Such tying up garters has never been seen.